1. Field of the Invention
The present invention relates to a wavelength control circuit and a wavelength control method of a light emitting device such as a semiconductor laser diode. More particularly, the invention relates to a wavelength control circuit and wavelength control method adapted to an optical transmission circuit of a wavelength multiplexing optical transfer apparatus.
2. Description of Related Art
It is known that the oscillation wavelength of a light emitting device such as a semiconductor laser diode changes according to the ambient temperature. As the ambient temperature increases, generally, the oscillation wavelength of a semiconductor laser diode shifts to the long wavelength side. On the contrary, as the ambient temperature decreases, the oscillation wavelength shifts to the short wavelength side.
In a wavelength control circuit of a conventional semiconductor laser diode, a semiconductor laser diode device (hereinbelow, called an xe2x80x9cLD devicexe2x80x9d) is disposed on an electronic heating and cooling device such as a Peltier cooler, a temperature sensing device is disposed near the LD device, and a uniform temperature is maintained while monitoring the temperature, thereby controlling the wavelength to be constant.
In an optical transfer system, however, an increase in transfer capacity has come to be demanded. In order to respond to the demand, a wavelength multiplexing optical transfer system for optically transferring a plurality of signal light having different wavelengths which are multiplexed is used.
In the wavelength multiplexing optical transfer system, in order to respond to a further increase in transfer capacity, the density of the signal light wavelengths further becomes higher. In a conventional wavelength control circuit which performs only a temperature control, a change in wavelength due to deterioration of a device cannot be monitored and controlled. The wavelength of the signal light from the LD device is therefore monitored and fed back for the temperature of the LD device, thereby controlling the wavelength. The structure disclosed in, for example, Japanese Unexamined Patent Application No. 7-249817 is known as a technique of monitoring the wavelength and driving an electronic heating and cooling device.
When a trouble such as a failure occurs for some reason in a wavelength monitor, however, the wavelength control circuit disclosed in the literature cannot control the wavelength of the signal light to a predetermined wavelength.
When the wavelength monitor is detached for maintenance of the wavelength monitor itself and is returned to an original temperature control loop, if the characteristics of an LD device are deteriorated at that time point, there is a problem such that a control is executed with an erroneous wavelength of signal light which is deviated from a desired wavelength.
It is an object of the invention to provide a wavelength control circuit and a wavelength control method of a semiconductor laser diode capable of normally controlling the wavelength not only in the case where the wavelength of an LD device changes but also at the time of maintenance of a wavelength monitor itself.
A wavelength control circuit of the invention comprises: a light emitting device for emitting signal light; a temperature sensing circuit for sensing a temperature around the light emitting device and outputting a temperature signal according to the temperature; and a wavelength deviation detecting circuit for detecting an oscillation wavelength of the signal light and outputting a second difference signal according to a wavelength deviation from a predetermined wavelength. The wavelength control circuit further comprises: an adding circuit for adding the first reference value with the second difference signal and outputting a signal obtained by the addition; a switching circuit for selectively outputting either the first reference value or the signal obtained by the addition; and a first difference signal generating circuit for comparing the first reference value or the signal obtained by the addition outputted from the switching circuit with the temperature signal and outputting a first difference signal. By a temperature control circuit, the temperature of the light emitting device is controlled by controlling a heating and cooling device on the basis of the first difference signal.
The switching circuit selects the first reference value after the start of light emission of the light emitting device and, after that, switches to select and output the signal obtained by the addition. The switching circuit selects the first reference value after the start of light emission of the light emitting device and, when a deviation of the first difference signal becomes smaller than a predetermined value, the switching circuit switches to select and output the signal obtained by the addition. Alternately, the switching circuit may select the first reference value for a predetermined time after the start of light emission of the light emitting device and switch to select and output the signal obtained by the addition after elapse of a predetermined time.
The wavelength control circuit of the invention further comprises a sample and hold circuit for sampling and holding the signal obtained by the addition and outputting the sampled signal, and the switching circuit selectively outputs the first reference value or the sampled signal. The sample and hold circuit comprises: an A/D converting circuit for converting the signal obtained by the addition into a digital signal; and a writable non-volatile memory for storing and holding the digital signal outputted from the A/D converting circuit.
On the other hand, the wavelength deviation detecting circuit includes: a wavelength monitor for outputting a wavelength signal according to the wavelength of the signal light; and a second difference signal generating circuit for comparing the wavelength signal with a second reference voltage and outputting a second difference signal. The wavelength deviation detecting circuit further comprises an optical branching device for branching a part of signal light emitted from the light emitting device and outputting the branched signal light to the wavelength monitor. Alternately, the wavelength deviation detecting circuit may output signal light emitted from the rear side of the light emitting device to the wavelength monitor.
A wavelength control circuit of the invention comprises: a light emitting device for emitting signal light; a temperature sensing circuit for sensing a temperature around the light emitting device and outputting a temperature signal according to the temperature; a wavelength deviation detecting circuit for detecting an oscillation wavelength of the signal light and outputting a second difference signal according to a wavelength deviation from a predetermined wavelength; an adding circuit for adding the second difference signal with the temperature signal and outputting a signal obtained by the addition; and a first difference signal generating circuit for comparing the first reference value with the addition signal and outputting a first difference signal. By a temperature control circuit, the temperature of the light emitting device is controlled by controlling a heating and cooling device on the basis of the first difference signal. In the construction of the invention as well, a sample and hold circuit for sampling and holding the signal obtained by the addition and outputting the sampled signal is provided and the switching circuit can selectively output the first reference value or the sampled signal.
The wavelength control circuit of the invention comprises a temperature control loop and a loop using a wavelength monitor. The temperature control loop monitors the ambient temperature of an LD device and drives an electronic heating and cooling device so that the temperature is constant on the basis of a difference signal between a signal value corresponding to the temperature and a reference signal. In the wavelength control loop, the wavelength of a light signal from the light emitting device is monitored by the wavelength monitor and the electronic heating and cooling device is controlled so that the wavelength is uniform. In the construction, at power on, the temperature is controlled by the temperature control loop, thereby controlling the wavelength of the LD device. On the other hand, after the temperature control loop is stabilized, a control is performed by using not only the temperature control loop but also the loop using the wavelength monitor. In the invention, a wavelength deviation detecting circuit for detecting a deviation in the signal light wavelength detected by the wavelength monitor as a difference signal is provided. It is also possible to perform a wavelength control by the temperature control loop by using a value obtained by adding the difference signal voltage and the reference voltage as a new reference voltage.
In the invention, it is also possible to provide a sample and hold circuit for sampling and holding a new reference voltage and supply a reference voltage held by the sample and hold circuit as a reference voltage of the temperature control loop. With the construction as well, the wavelength monitor can be maintained.